JP5517455B2 - Compensation of thermal suction in mass flow controllers - Google Patents

Description

  Thermal siphoning in a mass flow controller (MFC) can be referred to as continuous gas circulation caused by free residence between a heated thermal flow sensor and a bypass. As a result of heat suction, even when the actual output flow velocity is zero, an output signal with a non-zero flow velocity may be generated, which is similar to zero point drift. In some MFC designs, if the mass flow controller is installed vertically, the thermal suction effect may be more likely to occur and may vary in proportion to the molecular weight and pressure of the fluid whose flow rate is to be controlled. is there.

  In addition to causing the zero point calibration in the mass flow controller to shift, thermal suction can also cause calibration misalignment of the mass flow meter span or dynamic range of the mass flow controller.

  There is a need for a method and system that reduces or compensates for the effect of heat suction in a thermal mass flow controller.

  A thermal mass flow controller for controlling fluid flow includes a conduit configured to receive the fluid and a pressure sensor configured to measure the pressure of the fluid as the fluid passes through the conduit. A temperature sensor configured to measure the ambient temperature of the fluid, and a thermal sensor configured to generate an output representative of the fluid flow rate. The thermal mass flow controller further monitors the output from the thermal sensor, the pressure measured by the pressure sensor, and the ambient temperature measured by the temperature sensor to compensate for thermal sensor output deviations caused by thermal suction. A control system configured to regulate a flow rate of fluid in the conduit;

  A method for compensating for thermal suction in a thermal mass flow controller is described. The thermal mass flow controller includes a conduit configured to allow fluid flow between the inlet and outlet of the conduit and a thermal sensor configured to generate an output representative of the fluid flow rate. Including. The method involves monitoring the fluid pressure and fluid ambient temperature measurements, detecting thermal sensor output deviations caused by thermal suction, and compensating for the detected deviations at the inlet of the conduit. Regulating the flow rate of the fluid flowing in and flowing out from the outlet of the conduit.

  Systems and methods are described that significantly reduce or compensate for thermal suction in a thermal mass flow controller.

  1A and 1B schematically illustrate the operation of a typical thermal MFC that measures and controls the mass flow rate of the fluid, and can occur when the MFC is installed vertically as shown in FIG. 1B. Some thermal suction is also shown. FIG. 1A shows a thermal MFC mounted horizontally, while FIG. 1B shows the same thermal MFC as the MFC shown in FIG. 1A, but mounted vertically. In general terms, a thermal MFC can measure the mass flow rate of a fluid by using the thermal properties of the fluid to monitor the temperature change of the heating sensor as the fluid flows through the sensor. Thermal MFCs typically include a thermal mass flow meter that actually measures the mass flow rate of the fluid, and a control assembly that regulates the fluid flow rate so that the measured flow rate is equal to the desired flow set point (valve, And an electronic control circuit for controlling the operation of the valve). Typically, thermal MFC is considered to measure the mass flow rates of gases and vapors, but the flow rates of fluids other than gases and vapors can also be measured.

  Referring to FIG. 1A, a thermal MFC 100 includes a thermal mass flow sensor assembly 110, a conduit or flow body configured to receive fluid at the inlet 122 for measuring / controlling the flow rate, and a conduit 120. An internal bypass 130 may be included. Furthermore, the thermal MFC 100 can include a control system 150 that controls the operation of the valve 140 to control and supply the flow of fluid from the valve 140 and the outlet 123 of the conduit 120.

  The conduit 120 or flow body can define a main flow path or channel 124 and is at least partially in contact with the sensor receiving wall or sensor receiving surface 170. In the illustrated embodiment, the sensor receiving surface 170 is shown substantially parallel to the main flow path 124. Most of the fluid introduced into the MFC through the inlet 122 of the conduit 120 can pass through the main flow path 124. A relatively small amount of fluid is diverted by the bypass 130 and can pass through the thermal mass flow sensor assembly 110 and can reenter the main flow path 124 downstream of the bypass 130. The bypass 130 may be a pressure drop bypass that creates a pressure drop across the main flow path 124 and acts on the incoming fluid so that a relatively small amount passes through the thermal mass flow assembly. The inlet and outlet of the sensor tube 200 may coincide with the inlet and outlet of the main channel 124, so that the pressure drop across the bypass 130 can be the same as the pressure drop across the sensor tube 200.

  The thermal mass flow sensor assembly 110 can be attached to a sensor receiving surface 170 that forms at least a portion of the boundary of the conduit 120. The thermal mass flow sensor assembly 110 includes a thermal sensor 200 and a sensor tube configured to cause a diverted portion of incoming fluid to flow into a thermal sensor between the inlet 230 and outlet 240 of the sensor 200. A sensor heater configured to heat and a temperature measurement system configured to measure a temperature difference between two or more locations along the sensor may be included. Typically, the thermal sensor 200 may be a sensor tube. The sensor tube 200 may be a capillary with a thin wall and a small diameter, and may be made of stainless steel. However, different sizes, shapes, and materials can be used for the sensor tube 200.

  The sensor tube 200 includes a heat detection part 210 shown in FIG. 1A arranged in a horizontal direction parallel to the main flow path, and two legs 212 shown vertically in FIG. 1A. it can. A pair of resistive elements 250 and 251 can be placed in thermal contact with the heat sensing section 210 of the tube 200 at different locations along the heat sensing section 210 to provide a sensor tube heater and temperature measurement system. Can act as both part of As shown in FIG. 1A, the resistive elements 250 and 251 may be resistive coils, which are located at two locations along the tube thermal detector 210, one upstream (250) and the other. It is wound around the pipe 200 on the downstream side (251). The sensor tube 200 can be heated by applying a current to the resistive element. Thus, the resistive element can function as a tube heater.

  As the fluid introduced at the inlet of the sensor tube passes through the heated sensor tube at a substantially constant flow rate, the downstream resistive element 251 increases more heat than the upstream element 250. Can be transferred. The upstream coil 250 can be cooled by the fluid flow and provide a portion of its heat to the fluid flowing beside it, while the downstream coil 251 is heated and a portion of this heat applied to the fluid flow. Can be imported. As a result, a temperature difference ΔT can thus be produced between the two elements, thus providing a measure of the number of molecules of fluid (ie, fluid mass) passing through the sensor tube. By measuring the resistance change of each resistive element caused by the temperature difference, the temperature difference can be determined, resulting in an output signal from the mass flow meter as a function of the mass flow rate of the fluid. The output signal may be a voltage signal, but other types of signals can also be used for the output of the thermal flow sensor.

  When mounting the heat sensor tube in a certain direction, the heat gradient is generated in the sensor tube as the sensor tube is heated, especially as the heat sensor unit 210 is directed in a direction other than the horizontal direction. Aspiration may occur. As will be described below, heat suction can occur in vertically mounted MFCs even when the control valve is fully closed as shown in FIG. 1B.

  It is believed that as heat is transferred from the heated sensor tube surface to the gas, the temperature of the gas inside the heated sensor tube increases and the density of the gas decreases. The cold, dense gas in the bypass area is thought to be lowered by gravity. On the other hand, this is believed to raise the hot and light gas in the heated sensor tube. If the bypass area is sufficiently cold, the hot gas rising from the heated sensor tube cools and falls again. Thus, the continuous circulation of the gas inside the MFC is generally called heat suction, and occurs even when the control valve is completely closed so that the output flow becomes zero.

  As shown in FIG. 1A, when the MFC is mounted horizontally, it is considered that when the convection force is combined, the heat absorption is not seen. The horizontal section 210 of the sensor tube does not generate convection force, and the convection force generated by the two vertical legs 212 cancels out, so the total buoyancy is considered to be zero.

  As shown in FIG. 1B, when the MFC is rotated 90 degrees and mounted vertically, the sensor legs 212 can no longer generate convective forces. However, the heat detection unit 210 has a built-in heater coil, and it is considered that convection force is generated in this state. This is because in this case, the heat detection unit 210 is directed not vertically but vertically. Since the bypass is not heated, there is no convective opposition and no heat suction is considered to occur.

  In general, heat suction may cause a shift in the heat sensor output signal. Thermal suction may cause a zero shift, i.e., shift no output to a non-zero signal. Thermal suction can also cause a shift in the flow rate covered by the span or dynamic range, ie the relevant measurement range of the mass flow meter up to the intended maximum flow rate. As a result, the actual flow measurement may not be a function of the fluid inlet pressure and nature. The effect of heat suction on zero and span (dynamic range) is believed to increase with higher inlet pressure and gas density.

It is considered that the heat suction is governed by the Grashof number (G _r ). Typically, the Grashof number can be used to measure the severity of the heat suction problem. The Grashof number is the ratio of the buoyancy to the square of the viscous force and can generally represent free fluid heat transfer around the sensor tube.

Specifically, in one embodiment where the fluid is a gas, the Grasshof number G _r (no dimension) can be expressed by the following equation:

here,
g = gravity constant ρ = gas density α = gas thermal volume expansion coefficient T = gas temperature _Ta = ambient temperature d = inner diameter of sensor tube, and μ = gas viscosity.

  As can be seen from equation (1), the main factors affecting the heat suction are thought to include gas density and sensor tube diameter. Equation (1) shows that reducing the inner diameter of the sensor tube in the MFC generally reduces the effect of heat suction, but it may be difficult and impractical to manufacture a tube having such a small diameter, In addition, the dynamic range of the MFC design may be reduced. Although the mounting posture of the MFC has a great effect on heat suction, this effect is not subject to the Grashof number.

  Using the ideal gas law, the density ρ is expressed by the following equation.

  Where M is the gas molecular weight, P is the gas pressure, and R is the gas law constant. Substituting equation (2) into equation (1) yields:

In the formula (3), R, d, and g are constants, alpha, mu, and M are known for a given gas, the ambient temperature T _a is determined by the temperature sensor, T is dependent on T _a The gas pressure P is measured by a pressure sensor. That is, the Grashof number is determined.

The zero flow voltage signal _Vze and the maximum range flow voltage signal V _fs of the thermal flow sensor can be determined by a mathematical model using the Grasshof number and the MFC mounting orientation / position as follows.

  In the equations (4) and (5), Pos is a flag indicating the MFC mounting posture or the sensor tube orientation. Equations (4) and (5) can be further generalized as follows.

In the above formulas (6) and (7), P can be measured by a pressure sensor, Ta can be measured by _a temperature sensor, α, μ, and M are gas characteristics, and Pos is It is a predetermined factor. In other words, equations (6) and (7) show that V _ze and V _fs are known empirical functions of measured pressure and measured ambient temperature.

The thermal MFC can be calibrated with a predetermined fixed pressure P ₀ , a fixed ambient temperature T _a0 , and a fixed position Pos ₀ . That is, the calibration table of thermal sensor output voltage versus flow rate can be created as follows.

In the table above, the thermal sensor voltage V ₀ corresponds to the calibration pressure P ₀ , the calibration ambient temperature T _a0 , and the flow rate Q ₀ at the calibration position Pos ₀ .

The calibration table can be stored in the control system and used for flow rate calibration at a later time. If the inlet pressure, ambient temperature, and MFC mounting position do not change, the calibration table does not change and can be used directly to determine the flow rate based on the voltage output of the thermal flow sensor. However, if the inlet pressure and / or ambient temperature and / or the MFC mounting position change, the voltage output of the thermal flow sensor may shift due to heat suction. If this occurs, the calibration table is no longer accurate, i.e., is not valid. Since V _ze and V _fs vary according to equations (6) and (7), ie, are known to deviate, it is possible to compensate for the deviation in voltage output of the thermal flow sensor caused by the thermal suction effect. .

  A simple linear compensation for the voltage output of a thermal flow sensor can be formulated as follows:

1. Assume that the inlet pressure is P ₁ , the ambient temperature is T ₁ , the new MFC mounting position is Pos ₁ , and the thermal flow sensor output voltage is V ₁ .

2. The zero flow rate voltage is V _ze1 and the maximum scale flow rate voltage is V _fs1 , and calculation is performed according to Equation (6) and Equation (7).

3. Using the following formula (8) or formula (9), the thermal sensor output voltage V ₁ ′ compensated for thermal suction is calculated.

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4). A calibration table is searched to find the corresponding flow rate based on the compensated thermal sensor voltage output V ₁ ′.

  The above compensation methods include both linear and non-linear methods, and can be used to compensate for the effects of thermal suction on the thermal flow sensor voltage output based on equations (6) and (7). It is only one compensation method. By using equations (1) through (9) and by using observations on inlet pressure, ambient temperature, and MFC mounting position, the voltage output deviation of the thermal flow sensor caused by thermal suction is significantly reduced. Or you can compensate. That is, when a thermal MFC is mounted vertically by including a pressure sensor and a temperature sensor for measuring pressure and ambient / inlet temperature in the MFC, especially in a large-diameter thermal mass flow sensor, the thermal suction effect Can be greatly reduced. The control system may be calibrated to monitor pressure and temperature measurements to regulate fluid flow in a manner that significantly reduces or compensates for the voltage output of the thermal flow sensor caused by thermal suction. it can.

  FIG. 2A shows a thermal flow sensor caused by thermal suction by including pressure and temperature sensors on the thermal MFC and by monitoring pressure and temperature measurements to greatly reduce the thermal suction effect. FIG. 3 shows a thermal MFC 300 designed to significantly reduce or compensate for the output signal sigma.

In overview, the thermal MFC 300 is configured to receive a thermal mass flow sensor 310, a fluid measuring / controlling the flow velocity at an inlet 322, a conduit 320 having an outlet 323 through which the fluid flows, and It includes a bypass 330 that is internal to the conduit 320. The thermal MFC 300 further includes a valve 340 and a control system 370 that controls the operation of the valve 340 to regulate the flow rate of fluid flowing into and out of the inlet 322 of the conduit 320. The thermal flow sensor tube can have a zero flow voltage signal V _ze and a maximum memory voltage signal V _fs and can be a function of inlet fluid pressure, inlet fluid temperature, and orientation of the heated flow sensor tube.

Thermal MFC 300 further includes a temperature measurement system 360, a pressure sensor 380, and a temperature sensor 390. The temperature measurement system 360 is configured to measure a temperature difference between two or more locations along the heated flow sensor tube. The pressure sensor 380 is configured to measure the pressure P of the fluid as the fluid flows along the main flow path. Temperature sensor 390 is configured to measure the ambient temperature T _a of the fluid. The thermal flow sensor can be configured to convert the measured temperature difference into a voltage output signal V and can include a voltage detector configured to detect the voltage output signal.

  In response to the flow rate measurement by the thermal flow sensor, the pressure measurement by the pressure sensor, and the temperature measurement by the temperature sensor, the control system 370 regulates the flow rate of the fluid flowing into the outlet of the conduit 320 and out of the outlet, It is configured such that a deviation detected in the voltage output of the flow sensor can be substantially eliminated or compensated.

  The pressure sensor 380 can be attached either downstream or upstream of the thermal flow sensor 310. FIG. 2B shows a thermal MFC in which a pressure sensor 380 and a temperature sensor 390 are mounted on the thermal MFC, and the pressure sensor 380 is located downstream of the thermal flow sensor 310.

  In summary, a system and method have been described that significantly reduce the thermal suction effect in a thermal mass flow controller by compensating for deviations in the output signal of the thermal flow sensor using inlet pressure and ambient temperature information.

  While certain embodiments of the apparatus and method for substantially eliminating heat suction in MFC have been described, it will be appreciated that the concepts implied in these embodiments may be used in other embodiments as well. Yes. The protection of this application is limited only to the claims that follow.

  In these claims, references to singular elements are not intended to mean "one" unless specifically stated so, but "one" It means “above”. Elements that are structurally and functionally equivalent to the elements of the various embodiments described throughout this disclosure and that are well known or will become known to those skilled in the art are hereby expressly incorporated by reference. It is intended to be included in the present application and is intended to be covered by the claims. Furthermore, nothing disclosed herein is intended to be dedicated to the public, whether or not such disclosure is explicitly recited in the claims. Any claim element is not explicitly stated with the phrase “means to do” or, in the case of a method claim, the phrase “step to do” Unless otherwise specified using 35U. S. C. It shall not be interpreted in accordance with the regulations of §112, section 6.

FIG. 1A schematically illustrates the operation of the thermal mass flow controller and the reduction in heat absorption. FIG. 1B schematically illustrates the operation of the thermal mass flow controller and the reduction of thermal suction. FIG. 2A shows a thermal MFC, in which a pressure sensor and a temperature sensor are mounted on the thermal MFC to reduce the heat suction effect. The pressure sensor is attached upstream of the thermal flow sensor. FIG. 2B shows a thermal MFC, in which a pressure sensor and a temperature sensor are mounted on the thermal MFC to reduce the heat suction effect. The pressure sensor is attached to the downstream side of the thermal flow sensor.

Claims (16)

A thermal mass flow controller for controlling the flow rate of fluid,
A conduit configured to receive the fluid;
A pressure sensor configured to measure the pressure of the fluid as it passes through the conduit;
A temperature sensor configured to measure an ambient temperature of the fluid;
A thermal sensor configured to generate an output representative of the flow rate of the fluid, wherein the output of the thermal sensor comprises a voltage output;
The output from the thermal sensor, the pressure measured by the pressure sensor, and the temperature measured by the temperature sensor are monitored to compensate for deviations in the output of the thermal sensor caused by thermal suction, and the fluid flow rate in the conduit A control system configured to regulate;
With
The thermal sensor is configured to generate a temperature difference when heated when the fluid passes through the heated sensor, and a temperature measurement system configured to measure the temperature difference; And a thermal sensor tube having a tubular outer shape, and the thermal sensor is configured to convert the measured temperature difference into the voltage output,
The control system calibrates the thermal sensor with a zero flow voltage V _ze representing a voltage output when the fluid flow is zero and a maximum scale flow voltage V _fs representing a voltage output when the fluid flow is at a maximum scale. Is composed of
V _ze and V _fs are known empirical functions of the pressure measured by the pressure sensor and the temperature measured by the temperature sensor;
V _ze and V _fs are further known empirical functions of the Grashof number G _r depending on the measured pressure and temperature,
The Grashof number G _r is _expressed by the following equation:

Where g is the gravitational constant,
α is the gas thermal volume expansion coefficient of the fluid,
T _a is the ambient temperature measured by the temperature sensor,
T is the temperature of the fluid, depending on the T _a,
d is the diameter of the thermal sensor tube;
M is the gas molecular weight of the fluid;
P is a pressure measured by the pressure sensor,
μ is the viscosity of the fluid,
R is the universal gas law constant,
A thermal mass flow controller. The thermal mass flow controller of claim 1,
V _ze and V _fs further thermal mass flow controllers, characterized in that depending on the orientation of the thermal sensor tube for the main channel. The thermal mass flow controller of claim 2,
The orientation of the thermal sensor tube is represented by the parameter Pos,
Here, V _ze and V _fs are respectively represented by known empirical functions f _ze (P, T _a , α, μ, M, Pos) and f _fs (P, T _a , α, μ, M, Pos), respectively. A thermal mass flow controller characterized by being represented. The thermal mass flow controller of claim 3,
The control system further includes a corresponding flow rate 0,. . . , Q ₀ ,. . . , And Q _fs0, a plurality of calibration values V _ze0,. . . V _0,. . . Configured to calibrate the thermal sensor output at a calibration pressure P ₀ , calibration ambient temperature T ₀ , and calibration orientation Pos ₀ by calculating and storing V _fs0 ;
The thermal mass flow controller, wherein the control system is further configured to compensate for a shift in the thermal sensor output caused by thermal suction based on the plurality of stored calibration values.   5. The thermal mass flow controller of claim 4, wherein the control system is further configured to perform interpolation to compensate for the shift caused by thermal suction. Mass flow controller.   6. The thermal mass flow controller according to claim 5, wherein the interpolation includes at least one of linear interpolation and nonlinear interpolation. The thermal mass flow controller of claim 4,
The control system is further configured to calculate the voltage output V ₁ of the thermal sensor compensated for heat absorption at each of the measured values P ₁ , TV ₁ , and V ₁ of the pressure, temperature, and output voltage. Using the empirical functions f _ze (P, T _a , α, μ, M, Pos) and f _fs (P, T _a , α, μ, M, Pos), the zero flow voltage V _ze1 and the maximum scale voltage _Configured to calculate V _fs1 ,
The control system is further configured to determine a flow rate compensated for thermal suction by retrieving the plurality of stored calibration values and determining a corresponding flow rate based on the calculated V ₁ ′. A thermal mass flow controller. The thermal mass flow controller of claim 7,
The control system is configured to compensate for deviations caused by thermal suction by performing linear interpolation;
A thermal mass flow controller, wherein the control system is configured to calculate V ₁ ′ with respect to V _ze1 and V _fs1 using the following equations:

  The thermal mass flow controller according to claim 1, wherein the pressure sensor is located upstream of the temperature sensor.   The thermal mass flow controller according to claim 1, wherein the pressure sensor is located downstream of the temperature sensor.   The thermal mass flow controller of claim 1, further comprising a bypass in the conduit, wherein the bypass redirects a portion of the fluid toward the input end of the thermal sensor. A thermal mass flow controller configured to limit fluid flow entering the inlet of the conduit.   The thermal mass flow controller of claim 11, wherein the bypass comprises a pressure drop bypass configured to generate a pressure difference between the thermal sensors. The thermal mass flow controller of claim 1, wherein the temperature measurement system comprises:
A pair of heat sensitive resistive elements, each having a resistance that varies as a function of the temperature of the element;
A measurement circuit configured to determine the temperature of each of the elements by measuring the resistance of each element;
A thermal mass flow controller comprising:   The thermal mass flow controller of claim 1, further comprising a heater configured to heat at least a portion of the thermal sensor.   15. The thermal mass flow controller of claim 14, wherein the heater comprises a pair of heating coils configured to resistively heat the heat sensing portion of the sensor when supplied with current. Characteristic thermal mass flow controller. A method for compensating thermal suction in a thermal mass flow controller that controls fluid flow, wherein the thermal mass flow controller is configured to allow flow of the fluid between an inlet and an outlet of the conduit And a thermal sensor configured to generate an output representative of the fluid flow rate;
Monitoring the measured pressure of the fluid and the ambient temperature of the fluid;
Detecting the deviation of the output of the thermal sensor caused by thermal suction;
Adjusting the flow rate of the fluid flowing into and out of the conduit inlet to compensate for the detected deviation;
With
The operation of adjusting the flow rate of the fluid includes:
The operation of calibrating the thermal sensor with a zero flow voltage V _ze representing a voltage output when the fluid flow is zero and a maximum scale flow voltage V _fs representing a voltage output when the fluid flow is a maximum scale, _ze and V _fs are known empirical functions of the pressure measurement and the ambient temperature measurement , and the output of the thermal sensor has a corresponding flow rate of 0,. . . , Q ₀ ,. . . , And Q _fs0, a plurality of calibration values V _ze0,. . . V _0,. . . V _fs0 is calculated and stored, and the calibration pressure P ₀ , calibration ambient temperature T ₀ , and calibration orientation Pos ₀ are compensated for by compensating for the thermal sensor output shift caused by thermal suction based on the plurality of stored calibration values. The zero flow voltage V _ze and the maximum graduation flow voltage V _fs are calibrated at, and the thermal sensor voltage output compensated for thermal suction at each of the measured values P ₁ , T ₁ , and V ₁ of pressure, temperature, and output voltage. To calculate V _{1 ′} , using the known empirical functions f _ze (P, T _a , α, μ, M, Pos) and f _fs (P, T _a , α, μ, M, Pos) Calculated, behavior,
Determining the flow velocity compensated for thermal suction by searching the plurality of stored calibration values and determining a corresponding flow velocity based on the calculated V _{1 ′} ;
Where, where
P is the pressure of the fluid;
T _a is the ambient temperature of the fluid,
α is the gas thermal volume expansion coefficient of the fluid,
μ is the viscosity of the fluid,
M is the gas molecular weight of the fluid;
Pos is the mounting posture of the thermal mass flow controller, or the orientation of the thermal sensor,
A method characterized by that.

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